Supply circuits, in particular switched mode power supplies, are well known in the art. Such supply circuits are used to power consumer and non-consumer products. An exemplary application is the powering of light-emitting diodes (LEDs) and/or organic light-emitting diodes (OLEDs) or other lighting systems, in particular LED strings used in professional LED lighting systems, in particular lighting systems, where a precise color control is required. Further applications are LED backlighting, effect and accent illumination used for consumer products like “living colors lamps” and furthermore color temperature adjustment in general illumination applications as well as homogenizing the light output of single LEDs of the multiple LED lamp with a single supply circuit. It should be noted that in the following, the term “LED” is used as a generic term for similar applications such as OLEDs or the like.
Supply circuits that are best suited and therefore are preferably used for the above-named applications are in particular discontinuous series resonant converter with a constant average current output, in the following denoted as DSRC-I. The functionality of this type of converter is commonly known, e.g., from WO 2007/148271 A2 or WO 01/45241 A1 and shall therefore not be explained in detail. DSRC-I converters provide the advantage of a constant average current output, furthermore, no current sensing and no current control loop is required. Consequently, losses caused by a current sensing are avoided, and the DSRC-I provides a high-efficient, compact and easy design compared to other commonly known supply circuits. Above all, it is open and short circuit proof.
The basic DSRC-I topology provides only one single output of the supply circuit, in order to power a single load circuit, which is coupled to the supply circuit. However, with regard to the application of the supply circuit in general illumination applications, it is desirable to not only have one but multiple outputs in order to power different load circuits, e.g., comprising different colored LED strings such as red, green and blue LED strings.
As an example, a “living colors lamp” needs to be able to emit light of a huge number of colors generated by individually dimming and mixing the emitted light of red, green and blue LEDs in a certain ratio. Thus, by mixing the base colors red, green and blue (RGB), more than 16 million colors may be calculative generated. Also with regard to backlighting of liquid crystal displays (LCDs), LEDs are increasingly becoming the technology of choice. Thereby, the supply circuit is the key to the quality of the backlight image. The driver i.e., the supply circuit must be capable of providing a satisfactory dimming range that is high brightness for daylight vision and low brightness for night vision.
The commonly used dimming method for a DSRC-I is to reduce the switching frequency. However, this method is limited and hence does not provide a satisfactory dimming range. Furthermore, reducing the switching frequency will influence all outputs of a DSRC-I having multiple outputs. Consequently, this method allows no individual dimming of single outputs and thus, individual dimming for example of red, green and blue light and consequently, does not permit color control.
There have been made attempts to realize individual dimming LED strings. As an example, WO 2008/110978 describes a method, which uses additional switches in the load circuit to bypass the load. However, bypassing the load has harmful influences on the converter stability and reduces the overall efficiency.
As can be seen from the above explanations, there is a need for providing a supply circuit as well as a device comprising said supply circuit permitting individual dimming of individual outputs of the converter without negative influences on the converter stability and having a compact and easy design.